drm/ttm: Fix a theoretical race in ttm_bo_cleanup_refs()

In theory, that function could release the lru lock between
checking for bo on ddestroy list and a successful reserve if the bo
was already reserved, and the function was called with waiting reserves
allowed.
However, all current reservers of a bo on the ddestroy list would
atomically take the bo off the list after a successful reserve so this
race should not have been hit, so no need to backport for stable.

This patch also fixes a case found by Maarten Lankhorst where
ttm_mem_evict_first called with no_wait_gpu would incorrectly
spin waiting for bo idle if trying to evict a busy buffer that
also sits on the ddestroy list.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>